1. Field of the Invention
This invention relates generally to vapor deposition apparatus and methods, and specifically to a fixture for vapor phase diffusion coating of airfoil components.
2. Discussion of the Prior Art
Turbine components such as airfoils require coatings such as aluminides for environmental protection or as bond coats. However, only a portion of the airfoil that is exposed to the hot gases of the environment requires such protection, the remainder of the airfoil not requiring such protection. Not only does the remainder of the airfoil not require such protection, coating of that portion of the airfoil is undesirable.
Turbine airfoils have an airfoil section which extend outward into the hot gases resulting from the combustion of fuel in the combustor portion of the engine. These hot gases provide the energy for operation of the engine and for thrust to propel a turbine powered vehicle. Because of the high temperatures and corrosive effects of such gases on the airfoil section, the standard practice has been to coat the airfoil portions of the turbine airfoils with protective coatings that provide insulation from the extremely high temperatures and environmental protection from the corrosive effects of the hot gases. The portion of the turbine airfoil opposite the airfoil portion is used to attach the airfoil to the disk or rotor part of the engine, which is not in the flow of hot gases and therefore is not in need of protection from heat or corrosive effects. This portion of the turbine airfoil often has the shape of a dovetail, which is assembled into dovetail slots on the disk or rotor portion of the engine. Hence, it is frequently referred to as the dovetail. The walls of the dovetail portion of the turbine airfoil contact the walls of the dovetail slots of the disk or rotor. After a long period of time or rotating at high speeds, the dovetail walls exhibit a fatigue-related phenomenon referred to as fretting. Fretting has been found to be exacerbated by coatings applied to the airfoil portion of the turbine airfoil. Thus, in order to achieve the desired properties in the various portions of the turbine airfoil to maximize the life of the turbine airfoil, it has been necessary to devise methods to properly coat the airfoil portion of the turbine airfoil without affecting the dovetail portion of the turbine airfoil.
One of the methods of providing the coating to the desired portion of the turbine airfoil has been to mask the portion of the turbine airfoil that does not require coating, that is to say, the dovetail, before inserting the turbine airfoil into a coating apparatus. Typically, two different types of masks are applied. One type of mask is a material that is inert to the metallic gases used in the coating process. A second type of mask is a material that is a getter for the metallic gases used in the coating process, that is to say, the gasses preferentially are deposited on the getter material than on adjacent exposed portions of the dovetail. Both types of masks can be applied as a coating, slurry or as a tape. The difficulty with coatings and slurries is that they can enter into cooling holes. They are difficult to remove from such passages and can block or restrict the flow of air through cooling passages causing hot spots on the turbine airfoil. They are also time-consuming to apply and are difficult to remove. The problem with tapes is that they may not be sufficiently adherent during the coating process, so that coating material can reach the turbine airfoil through seams that may open up during the coating process. Additional problems exist with gettering materials used as masks. They can become saturated and, hence, ineffective after a period of time. Additionally, because they getter the metallic vapor within the coating apparatus, additional time at a given vapor pressure or additional vapor pressure at a fixed time is required to coat the portions of the turbine airfoil that are desirably coated. Finally, if the gettering materials are not properly mixed and applied, they can react with the dovetail portion of the blades, causing the very problem they were intended to prevent.
Fixtures are used in certain applications to hold substrates as they are being coated. One such device is set forth in U.S. Pat. No. 4,485,789 to Brandolf. This patent describes an apparatus for holding a substrate as the substrate is being rotated. The substrate is rotated to provide uniform coating, as the coating method involves line of site application. Planetary motion of substrate improves coating uniformity. The device utilizes graphite bearing means between the substrate mounting head and the base support of the device to act as a sliding bearing surface between the head and the base member to prevent sticking and binding of the rotating parts, and to provide an electrical conductive path between the base support and the substrate mounting head.
What is needed is improved apparatus and method for coating turbine airfoils. The apparatus and method should permit coating application to only those portions of the airfoil requiring protective metallic coatings while protecting those portions not requiring a protective coating. Furthermore, to be effective, the improved apparatus and method must be reusable and capable of reducing the current cycle time to prepare a turbine airfoil for coating, while providing a coating at least of similar quality to coatings currently applied.
The present invention provides a fixture and a method to improve vapor phase diffusion coating of turbine components. The fixture has general usage for coating any article in which only a portion of the article must be coated and in which a portion of the article must be masked from the coating media.
The fixture includes a barrier for segregating an interior volume of the fixture from the remaining volume of the coating apparatus or from the coating atmosphere external to the barrier. The fixture simply permits the creation of a protective environment for a portion of the article that must not be fully exposed to the coating media, thereby providing a mechanical barrier between the coating atmosphere external to the barrier and the internal volume of the fixture. The portion of the barrier that is exposed to coating atmosphere should be comprised of a material that is inert to the metallic coating vapors. Ideally, the material should have a low coefficient of thermal expansion. The barrier includes at least one aperture of a preselected configuration. The configuration of the aperture should be substantially the same as a cross section of the article that is to be coated, but slightly larger. This size and configuration permits the article to be coated to be readily inserted into the fixture, with the cross section of the article itself forming part of the boundary or barrier between the internal volume of the fixture and the coating atmosphere, the portion of the article forming a part of the boundary having a portion of at least one surface facing inward, bounding the internal volume and not exposed to the coating atmosphere, and a portion of at least one surface facing outward and exposed to the coating atmosphere.
The internal volume also may include a holder to support at least one article, such as a turbine component. The sole purpose of the holder is to provide stability to the article undergoing coating during the coating operation. This may be an elaborate fixture designed to clamp and hold the article, or it may simply be a ledge that the article may rest upon. However, once assembled into the fixture so that the article is held by the article holder, the portion of the article that is to be coated projects outwardly from the fixture so that the portion of the article that is not to be coated lies within the internal volume of the fixture, the interior volume of the fixture being defined to include that portion of the fixture, upon assembly of an article for coating, not otherwise exposed to the coating atmosphere.
Because the aperture is designed to be slightly larger than a cross section of the article so that the article, such as a turbine airfoil, can be easily and readily assembled into it, small gaps will exist in the aperture at the interface between the article assembled into the fixture and the fixture itself. These small gaps may allow for the seepage of some coating material into the interior of the fixture. For many applications, the reduced seepage may provide for an adequate reduction in the amount of coating material deposited on the portion of the turbine component within the interior of the fixture. However, certain applications may require no deposit of coating material onto the turbine component. In these cases, the gaps must be sealed. In order to provide an effective seal to prevent the leakage of coating media through these gaps and into the interior volume of the fixture, means for sealing these regions of the aperture, the small gaps, between the article, now assembled into the article holder, and the fixture walls forming the barrier, may be applied. The means for sealing provide a positive seal between the internal volume of the fixture and volume external to the fixture, so that there is no leakage of coating into the internal volume of the fixture and deposition onto the article in that region. In this manner, an effective seal can be formed so that only the portions of the article external to the fixture are exposed to the coating media.
An advantage of the present invention is that the apparatus and method of the present invention expose only those portions of the article requiring protective metallic coatings to the coating media, reducing the exposure of other portions of the article to the coating media, or alternatively sealing those portions not requiring a coating completely from exposure to the coating media.
Another advantage of the present invention is that the fixture and method are reusable. Although the fixture requires special design to accommodate an article, the article is readily assembled into the fixture and a means for sealing is easily applied. Thus, the use of this fixture reduces the current cycle time required to prepare an article such as a turbine airfoil for coating.
Because the fixture is made from a material that is inert to metallic materials, it has the added advantage of a particularly long life, since coating media will not be deposited on its exterior surfaces, unlike prior art metallic fixtures which are not only subject to undesirable coating build-up, but also to distortion and cracking.
The present invention also eliminates the need to apply a masking material to the article. Not only is the application of such masking material time-consuming, but for articles having intricate small, detailed features, such as cooling holes in turbine airfoils, the use of such masking material tends to clog, block or restrict the openings which can negatively impact the purposes of such features. The present invention advantageously eliminates the use of masking materials, providing a positive seal to prevent exposure of portions of the article from the coating media without the use of materials which may otherwise adversely affect fine features of the article.
Improvements in manufacturing technology such as described above are the keys to increased performance and reduced costs for many articles. As an example, continuing and often interrelated improvements in processes and methods have resulted in improved efficiency in the both the repair of gas turbine engines as well as in the manufacture of new components for gas turbine engines.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.
Whenever possible, the same reference numbers will be used throughout the figures to refer to the same parts.